Reactor for plasma assisted treatment of gaseous

ABSTRACT

A plasma reactor ( 11 ) of the silent discharge or dielectric barrier type for treatment of a gaseous medium is provided with a layer of material ( 34 ) positioned to present a surface extending along at least part of the length of the gas flow path. Particulates or selected species are entrapped on the surface. A preferred electrode arrangement provides surface discharge in the plasma at the surface of the layer of material.

[0001] The invention relates to the plasma assisted treatment of gaseousmedia and more particularly to the treatment of gaseous media in whichparticulate material is entrained.

[0002] The invention has particular application for the plasma-assistedprocessing of gaseous media for the reduction of the emissions of one ormore of nitrogenous oxides, particulate including carbonaceousparticulate, hydrocarbons including polyaromatic hydrocarbons andsoluble organic fractions, carbon monoxide and other regulated orunregulated combustion products from the exhausts of internal combustionengines.

[0003] One of the major problems associated with the development and useof internal combustion engines is the noxious exhaust emissions fromsuch engines that can include one or more of nitrogenous oxides,particulate including carbonaceous particulate, hydrocarbons includingpolyaromatic hydrocarbons and soluble organic fraction, carbon monoxideand other regulated or unregulated combustion products. Increasinglysevere emission control regulations are forcing internal combustionengine and vehicle manufacturers to find more efficient ways of removingthese materials in particular from internal combustion engine exhaustemissions. Unfortunately, in practice, it is found that combustionmodification techniques which improve the situation in relation to oneof the above components of internal combustion engine exhaust emissionstend to worsen the situation in relation to the other. A variety ofsystems for trapping particulate emissions from internal combustionengine exhausts have been investigated, particularly in relation tomaking such particulate emission traps capable of being regenerated whenthey have become saturated with particulate material.

[0004] Examples of such diesel exhaust particulate filters are to befound in European patent applications EP 0 010 384; U.S. Pat. Nos.4,505,107; 4,485,622; 4,427,418; and 4,276,066; EP 0 244 061; EP 0 112634 and EP 0 132 166.

[0005] In all the above cases, the particulate matter is removed fromdiesel exhaust gases by a simple, physical trapping of particulatematter in the interstices of a porous, usually ceramic, filter body,which is then regenerated by heating the filter body to a temperature atwhich the trapped diesel exhaust particulates are burnt off. In mostcases the filter body is monolithic, although EP 0 010 384 does mentionthe use of ceramic beads, wire meshes or metal screens as well. U.S.Pat. No. 4,427,418 discloses the use of ceramic coated wire or ceramicfibres. A recent review by Cutler and Merkel, ‘A New High TemperatureCeramic Material for Diesel Particulate Filter Applications’ publishedin the SAE 2000 International Fall Fuels and Lubricants Meeting andExposition as SAE 2000-01-2844 compares the use of diesel particulatefilters made out of cordierite, silicon carbide and sodium zirconiumphosphate. These filters are in monolithic form. Filters have been usedin conjunction with additives such as the cerium oxide-based fueladditive Eolys that acts as a carbon combustion catalyst as described inWO 99/43102.

[0006] In a broader context, the precipitation of charged particulatematter by electrostatic forces also is known. However, in this case,precipitation usually takes place upon large planar electrodes or metalscreens.

[0007] GB patent 2,274,412 discloses a method and apparatus for removingparticulate and other pollutants from internal combustion engine exhaustgases, in which the exhaust gases are passed through a bed of chargedpellets of material, preferably ferroelectric, having high dielectricconstant. In addition to removing particulates by oxidation, especiallyelectrical discharge assisted oxidation, there is disclosed thereduction of NO_(x) gases to nitrogen, by the use of pellets adapted tocatalyse the NO_(x) reduction.

[0008] Plasma assisted gas processing reactors have also been proposedwhich operate in the so-called “silent discharge” mode. Such silentdischarges are produced in a gas between electrodes, between which thereis at least one dielectric layer or dielectric material arranged so thatthere is no possibility for a direct, that is metal-to-metal, dischargethrough the gas. Such silent discharge reactors are also referred to asdielectric barrier reactors.

[0009] A silent discharge reactor of this type is disclosed in U.S. Pat.No. 5,746,051, the reactor comprising a number of flat rectangularelectrodes, as well as the same number less one of flat rectangularinsulating plates interleaved between the electrodes in paralleltherewith. U.S. Pat. No. 5,746,051 also identifies a number of prior artdevices known for producing silent discharges.

[0010] WO 00/71866 discloses an apparatus for removing particulate andother pollutants from internal combustion engine exhaust gasses. Theapparatus is also a dielectric barrier reactor for the plasma-assistedprocessing of a gaseous medium in which the gaseous medium isconstrained to pass between the co-axial electrodes with axial, radialand circumferential gas flow component combinations which result in thegas flow being at least partially helical and/or spiral.

[0011] Such dielectric barrier reactors or other non-thermal plasmareactors including the reactors described herein can be used forapplications other than vehicle applications for the treatment ofcombustion products from the exhausts of internal combustion engines.These other applications include reforming of hydrocarbon-based liquidor gaseous fuels or fuel processing to produce hydrogen containingstreams and/or streams containing oxygenated fuels that can be used forapplications including fuel cell applications in vehicles or stationaryfuel cell applications, and for liquid fuel production by reforminggases such as methane.

[0012] In referring to applications for reforming liquid hydrocarbonfuels, it is to be understood that this refers to fuels that are liquidat normal ambient temperatures. Such fuels are converted to vapour formin the process for their treatment in a non-thermal plasma reactor.Further applications for which a non-thermal plasma reactor can be usedare conversion of nitrogen oxides including nitric oxide to nitric acidso that the acid solution can be used in the preparation andpurification of inorganic oxides, in particular uranium oxides.

[0013] A problem which arises with plasma assisted gas processingreactors which include a bed of pellets of a high-dielectric constantmaterial, such as those exemplified in specification GB 2 274 412, isthat localised variations in the electric field in the pellet bed canoccur, possibly leading to regions of the pellet bed in which theelectric field is insufficient to enable a plasma to be established in agaseous medium flowing through the pellet bed of the reactor.

[0014] Reactors of the silent discharge type (such as disclosed in U.S.Pat. No. 5,746,051) having an array of alternate polarity parallel plateelectrodes with intervening dielectric barriers are advantageous inproviding efficient plasma generation in the spaces betweenplates/barriers. However, a problem with this type of reactor is that,where the gas contains particulate matter, such as soot, the residencetime of the gas in a reactor of practical proportions is insufficientfor plasma assisted oxidation of the particulate matter to be completed.It is therefore necessary to trap the particulate material, which can bedone by introducing a packed bed of pellets into the gas flow space, thepacked bed acting as a filter on which particulate matter is trapped.However, one then faces again the problem mentioned above of regionswithin the packed bed in which the electric field is insufficient toenable a plasma to be established in a gaseous medium as it flowsthrough that region. Also deposition tends to be concentrated at the gasinlet region.

[0015] It is an object of the present invention to provide an improvedreactor for the plasma assisted processing of gaseous media.

[0016] According to the invention there is provided a reactor of thesilent discharge or dielectric barrier type for plasma assistedtreatment of a gaseous medium, which reactor comprises at least one pairof electrodes between which is an intervening dielectric barrier layerdefining between at least one of the electrodes and the dielectricbarrier layer at least one gas flow path through which gaseous mediummay pass, such that, in use, a plasma discharge is generated in thegaseous medium by application of an appropriate electrical potentialacross the or each electrode pair, a layer of material between theelectrodes, which layer may comprise the said dielectric barrier layeror may be separate therefrom, and which provides a surface over whichthe gaseous medium flows, the surface extending along at least part ofthe length of the said gas flow path, and means for causing entrapmenton the said surface of selected species or particulate matter in thegaseous medium.

[0017] In one arrangement according to the invention, the said means forcausing entrapment comprises a source of direct, alternating or pulsedvoltage applied so as to electrostatically trap on the said surfaceparticulate matter in the gaseous medium. For electrostatic trapping inthis way, it may be necessary to pre-charge the particulate matter byexposing the gaseous medium carrying the particulate matter to anelectric field.

[0018] Preferably the said layer of material is provided by permeablefilter material in sheet form and the said gas flow path is such thatthe gaseous medium is directed to flow through the sheet form filtermaterial, whereby selected species or particulate material in thegaseous medium is entrapped on the surface thereof. One or both sides ofthe said layer of material may be provided with a coating. The coatingmay be such as will act as a catalyst for the removal of nitrogenousoxides or for the removal of carbonaceous material. Alternatively thecatalyst on one surface may catalyse one reaction (such as reduction ofnitrogenous oxides) and the catalyst on the other surface may catalyse adifferent reaction (such as oxidation of carbonaceous material). Or thecoating or coatings may contain a mixture of catalysts.

[0019] When using the reactor, an electrical power supply is connectedto apply a high voltage alternating or pulsed or direct currentelectrical potential or combination of these potentials to theelectrodes so as to generate a plasma in a gaseous medium between theelectrodes and such as to act upon the said surface, thereby to assistin the oxidation of particulate matter entrapped thereon or thepromotion of reactions including catalytic reactions involving entrappedselected species. Catalytic materials may be present in the plasmaregion to aid in the removal of nitrogenous oxides and carbonaceousmaterial. For alternating potentials, triangular waves, sine waves,square wave, saw-tooth wave of the same or similar characteristics canbe used separately or in combination.

[0020] According to a preferred feature of the invention, the electrodesare so arranged as to promote surface discharge to take place along thesaid surface on which particulate matter or selected species isentrapped. Such surface discharge can be arranged to providesubstantially uniform plasma treatment of particulate matter or selectedspecies trapped on the surface. A bulk or volume plasma discharge canalso be present in addition to the surface discharge.

[0021] For the promotion of surface discharge, in a preferredarrangement according to the invention, an array of discrete electrodesis positioned close to the said surface, each discrete electrode beingelectrically insulated from neighbouring electrodes, whereby applicationacross each successive pair of the said discrete electrodes of a highvoltage alternating or pulsed or direct electrical potential to generateplasma in gaseous medium adjacent the said surface causes surfacedischarge across the said surface between the electrode pairs. In thisway the electrical power in the plasma is applied to the surface wherethe particulate material or selected species is trapped so that thesurface discharge enhances reactions thereof and/or the catalytic actionof the surfaces for treating the gaseous media.

[0022] Preferably, the gas flow path extends over and through the fullextent of the surface acted upon by the said array of electrodes and afurther continuous electrode is positioned spaced apart from the surfaceon the side thereof remote from the said array of electrodes, so thatthe said gas flow path passes between the said continuous electrode andthe said surface, the said continuous electrode in use being held atground or at a fixed potential relative to ground, and the pulsed oralternating or direct current potential applied across the neighbouringpairs of the electrodes in the array being respectively above and belowthat of the continuous electrode, whereby, in addition to surfacedischarge across the said surface between adjacent pairs of electrodesin the array there is a volume plasma discharge between the saidcontinuous electrode and the said array of electrodes.

[0023] According to one aspect of the invention a high electric field inthe surface region of the filter material extends into the boundarylayer region of the gas flow, a region where the gas flow ispseudo-laminar.

[0024] To provide for efficient treatment of a large gas flow volume, astacked arrangement of components provides a plurality of gas flow pathsin parallel, each component in the stacked arrangement comprising a gasflow path defined between a said continuous electrode, a said surfaceand a said array of discrete electrodes positioned close to the saidsurface, the gas flow path extending over and through the full extent ofthe surface acted upon by the said array of discrete electrodes.

[0025] Preferably the or each dielectric barrier layer is in intimatecontact with an electrode. By intimate contact we mean that thedielectric layer is either chemically bonded to the, usually metal,electrode or that the dielectric layer is in physical contact with theelectrode. This intimate contact reduces any losses due to discharges orcorona in any gaps between the electrode and barrier. This means thatthe power applied to the reactor is more efficiently coupled forgenerating a plasma discharge for processing of the gaseous media.Reducing these losses increases the efficiency of the dielectric barrierreactor and power supply system for a vehicle application and so reducesthe power requirement and possible fuel penalty which are keyconsiderations for any design. Reducing such losses also helps minimiseelectromagnetic emissions improving electromagnetic compatibility forvehicle applications. The physical contact between the electrode anddielectric material may be enhanced by depositing a metallised layer asa coating onto the dielectric material. The layer can be depositedelectrolytically or by screen printing and can be made of, but is notrestricted to, a suitable conducting material such as silver, nickel orcopper. The metallised layer can also constitute the electrode. By wayof an example an intermediate layer of molybdenum/manganese can first bedeposited on the dielectric material and fired on at around 1400° C.causing some of the metal to diffuse into the dielectric materialsurface. A conducting layer, for example nickel is then deposited ontothe molybdenum/manganese so that the nickel makes a uniform contact withthe diffused metal layer on the dielectric material. In this way astrong, intimately bonded, metallised layer on the dielectric materialis achieved between the nickel-metal electrode and the dielectricmaterial. We have also found that enhanced formation of dischargestreamers into the gas flow path and emanating from dielectric surfacesis achieved by providing on those surfaces exposed to gas an isolateddiscontinuous metallic film, preferably less than 10 μm in thickness andnot connected to an external source of electric potential. Thediscontinuous metallic film may be in the form of spots or dots of metaland can be present on one or more of the dielectric barriers. The metalprovides a richer source of electrons than the dielectric material andhas the effect of increasing the number of discharge points emanatingfrom one or more of the barriers and thereby increasing the efficiencyof treatment of exhaust gases.

[0026] In a reactor configuration in which the said layer of material isprovided by permeable filter material in sheet form and the said gasflow path is such that the gaseous medium is directed to flow throughthe sheet form filter material, the gas flow path on the downstream sideof the layer of filter material may be packed with a gas permeable bedof an active material. The permeable filter material in sheet form maybe the active material that may be selected to act as a catalyst withrespect to the treatment of the gaseous medium that passes through thereactor. Further, the material of the said layer of filter material maybe selected to act as a catalyst, or a catalytic material may be coated,or otherwise incorporated, on or in the layer of filter material. Thematerial of the layer or coating may be selected to be catalytic toincrease the efficiency of oxidation of particulates and/or thereduction of nitrogenous oxides to nitrogen present in the exhaust frominternal combustion engines. However, a preferred arrangement is for thelayer of filter material to be such as to catalyse the oxidation ofparticulates trapped thereon and for the gas flow path downstream of thelayer of filter material to be packed with material which catalyses thereduction of nitrogenous oxides to nitrogen. The layer of filtermaterial may also be catalytic for the reduction of nitrogen oxides. Inaddition components of the reactor such as the dielectric barrier layermaterial can be coated, partially or fully with a material or materialsor combinations of materials to act as a catalyst for removal ofnitrogenous oxides and carbonaceous material.

[0027] It should be appreciated that a packing material, layer materialor coating that is not catalytic for the oxidation of carbonaceousparticulate or reduction of nitrogenous oxides, for example by thermalmechanisms, may develop catalytic properties for these processes whenexposed to a plasma. This may be due, for example to activation byoxygen atoms or other plasma-generated free radicals or activation byplasma generated species such as activated hydrocarbons as described inWO 99/12638, organo-nitrogen or activated organo-nitrogen species and ornitrogen dioxide. Catalytic or non-catalytic material properties can befurther augmented by the electric field or by other charged speciespresent in or adjacent to the plasma region.

[0028] The gas permeable bed of active packing material, can be in theform of spheres, pellets, extrudates, fibres which can be in the form ofa mat, felt or blanket or vacuum formed shape or a shape made from acontinuously-wound fibre, sheets, wafers, frits, meshes, coils, foams,membrane, ceramic honeycomb monolith or granules or as a coating on anyof the above shapes or contained within a dielectric, polymeric ormetallic material in any of the above shapes or as a combination of morethan one of the aforementioned forms of packing material. When the gaspermeable bed of active packing material is fabricated from fibres, thelatter can be sintered together for example when in the form of a mat inorder to limit the release of loose fibres from the packing materialinto the exhaust gases.

[0029] Examples of oxidation catalysts such as carbon combustioncatalysts for use in the layer of filter material are alkali-metal saltssuch as lithium nitrate described in GB 2 232 613 B, cerium oxide,alkali-metal doped lanthanum oxide-vanadium oxide such aslanthanum-caesium-vanadium pentoxide in addition to the activematerials, alkali metal metavanadates, alkali metal pyrovanadates,perovskites including layered perovskites and combinations of thesematerials. Some of these combustion catalysts such as perovskites cansimultaneously remove both nitrogen oxides and carbonaceousparticulates. Examples of perovskites are La₂CuO₄,La_(1.9)K_(0.1)Cu_(0.95)V_(0.05)O₄, La_(0.9)K_(0.1)CoO₃,La_(0.6)Cs_(0.4)CoO₃ and La_(0.8)Sr_(0.2)Mn_(0.5)Cu_(0.5)O₃. The mode ofoperation of such catalysts is described in specification WO 00/43102.The use of a carbon combustion catalyst can reduce the powerrequirements to the plasma reactor for treating carbonaceous particulatematerial and reduce the volume of active material required. Otherexamples of oxidation catalysts are manganese oxide doped aluminas,whose synthesis has been described, for example, in U.S. Pat. No.5,880,059, lanthanide oxide doped tin oxide for example lanthanum oxidedoped tin oxide, platinum containing molybdenum oxide and platinumcontaining alumina.

[0030] The exhaust may also contain a chemical additive acting as acarbon combustion catalyst that is either present initially in the fuelor added separately to the exhaust and whose function is to lower thecombustion temperature and/or increase the rate of removal ofcarbonaceous material. Carbon combustion catalyst can be encapsulatedwithin or bound to a fugitive additive that chemically decomposes duringor shortly after fuel combustion thus releasing the additive into thefuel or exhaust.

[0031] Examples of packing material for catalysing the reduction ofnitrogenous oxides are activated alumina such as gamma alumina, or alphaalumina or zirconium dioxide or titanium dioxide, silver aluminate,silver doped alumina, spinels, vanadium pentoxide, metal-doped and metaloxide-doped or exchanged inorganic oxides such as cobalt oxide-dopedalumina, and metal-doped zeolites. Zeolites are particularly usefulmaterials for the reduction of nitrogenous oxides. Examples of zeolitesare those known as ZSM-5, Y, beta, mordenite all of which may containiron, cobalt or copper with or without additional catalyst promotingcations such as cerium and lanthanum. Other examples of zeolites arealkali metal containing zeolites in particular sodium-Y zeolites thatare particularly useful for treatment of nitrogenous oxides. Anotherzeolite especially useful for removal of nitrogenous oxides isferrierite with silica to alumina mole ratios up to thirty andcontaining up to 10 percentage by weight of silver. It should beappreciated that zeolites, depending on their chemical composition, canalso exhibit oxidative properties towards the gaseous and particulateprocessing reactions.

[0032] An additive may be required to improve the process of oxidationand/or reduction of the gaseous media constituents in combination withthe layer of filter material and the packing material. For example whena nitrogen containing species such as ammonia, urea or cyanuric acid isused for nitrogenous oxide reduction, a particularly useful catalyst isvanadium pentoxide-titanium dioxide. Hydrocarbons are another suitableadditive either added separately or residually derived from combustionfuel to promote processes such as selective catalytic reduction ofnitrogenous oxides.

[0033] Specific constructions of reactor embodying the invention willnow be described by way of example and with reference to the drawingsfiled herewith, in which:

[0034]FIG. 1 shows a reactor in cross-section taken transverse to thedirection of incoming and out-flowing gas,

[0035]FIG. 2 shows the reactor of FIG. 1 in cross-section taken parallelwith the direction of incoming and out-flowing gas,

[0036]FIG. 3 illustrates a modification showing a part of a reactor incross-section taken parallel with the direction of incoming andout-flowing gas, and

[0037]FIG. 4 illustrates, in a cross-sectional view corresponding tothat of FIG. 2, a similar modification to the reactor of FIGS. 1 and 2.

[0038] Referring to FIGS. 1 and 2, the reactor 11 comprises arectangular box enclosure formed between a ceramic cover plate 12 and aceramic base plate 13. In this example, there are four electrode plates14, 15, 16, and 17, each having an extension to provide an electricalcontact, those, 18, 20, from electrode plates 14, 16 being on one sideof the reactor 11, whilst the electrical contacts 19, 21 from theelectrode plates 15, 17 are on the other side of the reactor 11.

[0039] Each electrode plate 14, 15, 16, 17 is secured with adhesivebetween respective pairs of ceramic plates 12, 22; 23, 24; 25, 26; and27, 13. Where the electrode plates finish short of the edges of theceramic plates, the space between respective ceramic plates is filledwith ceramic adhesive, as for example referenced at 29 (FIG. 2). Theceramic plates serve as dielectric barrier layers for their associatedelectrode plates. Alumina is a suitable material for the ceramicdielectric barrier material. Aluminium nitride can also be used.However, as an alternative, a glass-ceramic may be used or a micaceousglass such as MICATHERM as described in publication WO 99/20373. Thedielectric barrier material can be a catalytic material, or contain acatalytic coating in or on its surface. The catalytic material can beproduced by ion-exchange, doping, deposited by wet chemical techniquessuch as sol-gel processing, incipient wetness, by sputtering or bychemical vapour deposition or by thermal spraying for example by plasmaspraying or by physical and chemical vapour deposition. The type ofdielectric barrier material or coating, be it catalytic or non-catalyticcan be selected from those described for the packing material.

[0040] Ceramic spacers, one of which is referenced at 31, serve to holdthe electrode plates spaced apart and also to form side walls of therectangular box enclosure. At each end of the reactor 11, openings areprovided at 32 a, 32 b and 32 c for gas flow into the reactor (asillustrated by the arrows A in FIG. 2), and at 33 a, 33 b, 33 c for gasflow out of the reactor (as illustrated by the arrows B in FIG. 2).

[0041] The ceramic spacers (such as 31) are arranged also to support, ineach of the three spaces between the four electrode plates, a layer ofmaterial 34 a, 34 b, and 34 c, each of which, in this example, is asheet of gas permeable filter material providing a surface which extendsalong the length of the respective gas flow path (32 a, 32 b and 32 c).This permeable filter may be in the form of different shapes in order tooptimise the trapping of particulate material. It can have indentationswith a square wave-form as shown in FIGS. 1 and 2 but other shapes suchas mesh form, or indentations with a triangular form can be used.

[0042] In operation, the reactor 11 is connected to the source ofgaseous medium to be treated (for example the exhaust gases of a motorvehicle) so that the gaseous medium flows into the reactor through theopenings 32 a, 32 b, 32 c in the direction of the arrows A in FIG. 2.The gaseous medium is then forced to flow through the layers of filtermaterial 34 a, 34 b, 34 c, in order to exit from the reactor 11 via theopenings 33 a, 33 b, 33 c.

[0043] Particulates entrained in the gas flow are trapped on the layersof filter material and are oxidised by the plasma. The filter materialmay alternatively or additionally be such as to trap selected speciesfrom the gaseous medium.

[0044] An electrical power supply (not shown) is connected to applybetween the pair of electrode plates 15, 17 and the pair of electrodeplates 14, 16 a pulsed or alternating potential or direct current of theorder of hundreds of volts, kilovolts to tens of kilovolts and (in thecase of pulsed or alternating potential) repetition frequencies in therange 50 Hz to 50 kHz. It is possible to make the discharge gaps narrowfor this embodiment of the invention, that is of the order of 0.5 to 5millimetres, the dimension transverse to the gap dimension being of theorder of 10 to 200 millimetres. A significant advantage follows from thesmall gap dimension in that the applied voltage required to generate aplasma discharge is reduced and may typically be of the order of 5kilovolts or less applied across each gas flow path. The plasma,together with any catalytic material incorporated in or on the layer offilter material, acts to oxidise particulate trapped on the filtermaterial. The reactor 11 can operate above or below atmospheric pressureand from −40° C. to 400° C. and is thus able to operate at temperaturesrepresentative of those found in diesel exhausts from internalcombustion engines.

[0045]FIG. 3 illustrates a single channel of a modified form of reactor41. Ceramic cover plate 42, upper electrode plate 44, and ceramicdielectric barrier layer 52 are similar to the corresponding componentsin the example of FIGS. 1 and 2. The opposed electrode structuredefining the opposite side of gas flow path 45, however, consists ofthree ceramic dielectric barrier layers 53, 54, 55 between which aresandwiched two electrode grid structures, spaced from one another by thedielectric layer 54. As shown in FIG. 3, the small plate like components46 of the upper (as seen in the FIG. 3) electrode grid structure arelaterally displaced with respect to the corresponding small plate likecomponents 47 of the lower electrode grid structure. The thickness ofthe various components is exaggerated in all figures including FIG. 3,the dielectric barrier layers 52, 53, 54, 55 being of the order of 0.1-5millimetre thick, preferably 1 mm thick.

[0046] In this example, electrode plate 44 is held at a fixed potential,most conveniently ground potential, while the electrode grid structures46 and 47 are connected to a source of alternating or pulsed or directpotential with respect to the electrode plate 44 such that thepotentials applied to the electrode grid structures 46 and 47 are ofopposite polarity. The effect of this is to create, in addition to avolume discharge across the gap represented by the gas flow path 45, asurface discharge along the surface of the dielectric layer 53. Thevolume discharge is indicated by arrows such as 48 and the surfacedischarge is indicated by the arrows 49 and is a consequence of theelectric fields set up by the difference in potential between theelectrode grid structure 46 and the electrode grid structure 47.

[0047] The principal feature of this arrangement is the generation of asurface discharge on the dielectric layer 53, since this surfacedischarge is particularly effective for assisting the oxidation ofparticulates trapped on the surface. In order to cause particulates in agaseous medium flowing through the path 45 to be trapped on the surfaceof the dielectric barrier layer 53, a direct voltage between theelectrode plate 44 and the electrode grid structures 46 and 47 togetheris superimposed in addition to the alternating or pulsed potentialsapplied separately to the electrode grid structure 46 and electrode gridstructure 47. This superimposed additional direct voltage is chosen soas electrostatically to charge particulates and thereby encourage theirprecipitation on the surface of the dielectric layer 53. The geometry ofthe surface in the region of the surface discharge can be arranged totrap particulate by direct impingement, for example by serrating thesurface between the buried electrode structures.

[0048]FIG. 4 illustrates how this principle may be applied to a reactorconfiguration similar to that shown in FIGS. 1 and 2. In FIG. 4,components corresponding to those shown in the example of FIGS. 1 and 2are referenced with the same reference numerals.

[0049] In the example shown in FIG. 4, the volume plasma discharge isset-up between electrode plates 14 and 15, between the electrode plates15 and 16, and between electrode plates 16 and 17 in the same way as inthe example of FIGS. 1 and 2. The differences are that the layers offilter material 34 a, 34 b, 34 c have the form of a continuous sheet onone side with upstanding, square-section, ribs on the other. Extendingbetween and along the length of the ribs is an array of insulated metalrod electrodes 61 connected so that adjacent rod electrodes 61 haveopposite polarity electric potential applied to them. These rodelectrodes 61 serve to provide a surface discharge on the surfaces ofthe layers of filter material 34 a, 34 b, 34 c.

[0050] The invention is not restricted to the details of the foregoingexamples. For instance, the layers of filter material 34 a, 34 b, 34 cneed not necessarily have the particular form shown, but may compriseany suitable layer of gas permeable material providing a surface onwhich particulate matter is trapped as gaseous medium passestherethrough. For example, a slice of permeable monolithic foam may beused.

[0051] Some or all of the dielectric barrier layers 22-27 may beapertured to provide an array of so-called triple junctions between themetal electrode, dielectric material and gas which are effective todecrease plasma ignition voltage. The work function of the metal isreduced at the point of contact between the metal, insulator and air,the triple junction, due to penetration of the electric field into theinsulator (the dielectric material) but not the metal. Electrontunnelling occurs from the metal to the conduction band of the insulatorand electrons can be emitted from both the metal and the insulator.Electrical discharge takes place at lower voltage in the compositeelectrode so that higher plasma currents (plasma energy per litre ofplasma) can be obtained. This triple effect increases the number ofdischarges per unit length of reactor. A similar effect may be achievedwith the configuration shown in the example of FIG. 4 by inverting thelayers of filter material 34 a, 34 b, 34 c together with theirassociated rod electrodes 61, and arranging for the volume dischargeelectric potential to be applied between the array of rod electrodes 61and the electrode plate (14, 15,16) now facing the side of the layer offilter material (34 a, 34 b, 34 c respectively) in which the slotscontaining the electrode rods 61 are open, thus providing an array ofeffective triple junctions between the filter material (which has, forthis purpose, to be of suitable dielectric material), the rod electrodes61 and the incoming gaseous medium.

[0052] Application of a magnetic field, or magnetic fields, to encouragespiral motion of electrons and ions in the gas plasma formed in the gasflow paths further improves the effectiveness of the plasma andefficiency of the reactor. This is because of the longer path followedby the discharge between electrodes as a result of the spiralling motionof electrons and ions.

[0053] As mentioned above, enhanced formation of discharge streamersinto the gas flow path and emanating from dielectric surfaces isachieved by providing on those surfaces exposed to gas an isolateddiscontinuous metallic film, preferably less than 10 μm in thickness andnot connected to an external source of electric potential. Thediscontinuous metallic film may be in the form of spots or dots (of anycircumferential shape, such as circular, rectangular or irregular, evenfractal) of metal and can be present on one or more of the dielectricbarriers. Such added metallic sources of discharge points can also beprovided on or in dielectric packing material, where this is alsoprovided in the gas flow path between the electrode. Thus, for exampledielectric packing material in the form of spheres or other shapes maybe provided with surface deposits of thin film metal dots or spots.Alternatively, the packing itself may contain a distribution of metallicspheres (or other shapes) in amongst the dielectric packing. Suchsurface metal dots or spots are also advantageous on a dielectricsurface for which provision has been made for promoting surfacedischarge, such as the surface of dielectric layer 53 in FIG. 3 or thesurfaces of the filter material 34 a, 34 b, 34 c of FIG. 4. However, itwill be appreciated that the location and dimensions of the metal dotsor spots must be such that there is not formed a metallic path whichwould effectively short out the surface discharge path between theelectrodes, such as 46, 47 in FIG. 3 or adjacent rods 61 in FIG. 4.

[0054] The reactor embodiments described may be installed as part of anemissions control system employing catalysts or other emission controldevices for the plasma assisted treatment of the exhaust gases frominternal combustion engines. These other emission control devices maycomprise exhaust gas recirculation (EGR), variations in ignition timing,fuel ignition timing and fuel injection pulse rate shaping.

1. A reactor of the silent discharge or dielectric barrier type forplasma assisted treatment of a gaseous medium, which reactor (11;41)comprises at least one pair of electrodes (14,15; 16,17; 44, 46/47)between which is an intervening dielectric barrier (22,23,24,25,26,27;52,53)defining between at least one of the electrodes and the dielectricbarrier layer at least one gas flow path (32,33; 45) through whichgaseous medium may pass, such that, in use, a plasma discharge isgenerated in the gaseous medium by application of an appropriateelectrical potential across the or each electrode pair (14,15; 16,17;44, 46/47), a layer of material (34;53) between the electrodes, whichlayer may comprise the said dielectric barrier layer or may be separatetherefrom, and which provides a surface over which the gaseous mediumflows, and means for causing entrapment on the said surface of selectedspecies or particulate matter in the gaseous medium characterised inthat the said surface extends along at least part of the length of thesaid gas flow path (32,33; 45).
 2. A reactor as claimed in claim 1,further characterised in that the said means for causing entrapmentcomprises a source of direct, alternating or pulsed voltage applied soas electrostatically to cause entrapment on the said surface ofparticulate matter in the gaseous medium.
 3. A reactor as claimed inclaim 1 or claim 2, further characterised in that the said layer ofmaterial (34) is provided by permeable filter material in sheet form andthe said gas flow path (32,33) is such that the gaseous medium isdirected to flow through the sheet form filter material (34), wherebyselected species or particulate material in the gaseous medium isentrapped on the surface thereof.
 4. A reactor as claimed in any of thepreceding claims, wherein an electrical power supply is connected toapply a high voltage alternating or pulsed or direct electricalpotential to the electrodes (14,15;16,17;44, 46/47) so as to generate aplasma in gaseous medium between the electrodes and such as to act uponthe said surface, thereby to assist in the oxidation of particulatematter or reactions involving the selected species entrapped thereon. 5.A reactor as claimed in any of the preceding claims, furthercharacterised in that a surface of at least a part of the dielectricbarrier layer or layers (22,23,24,25,26,27;52,53) exposed to gas in thegas flow path (32,33; 45) is provided with discontinuous, isolated, thinmetal film deposits.
 6. A reactor as claimed in claim 5, furthercharacterised in that the thin film metal deposits are in the form ofdots or spots on the surface on which they are deposited.
 7. A reactoras claimed in any of the preceding claims, further characterised in thatthe electrodes(46,47; 61) are so arranged as to promote surfacedischarge to take place along the said surface on which the selectedspecies or particulate matter is entrapped.
 8. A reactor as claimed inclaim 7, further characterised in that an array of discrete electrodes(61) is positioned close to the said surface, each discrete electrode(61) being electrically insulated from neighbouring electrodes (61),whereby application across each successive pair of the said discreteelectrodes (61) of a high voltage alternating or pulsed or directelectrical potential to generate plasma in gaseous medium adjacent thesaid surface causes surface discharge across the said surface betweenthe electrode pairs.
 9. A reactor as claimed in claim 8, furthercharacterised in that the gas flow path(32,33) extends over and throughthe full extent of the surface acted upon by the said array ofelectrodes (61) and a further continuous electrode (14,15,16,) ispositioned spaced apart from the surface on the side thereof remote fromthe said array of electrodes (61), so that the said gas flow path(32,33)passes between the said continuous electrode (14,15,16) and the saidsurface, the said continuous electrode (14,15,16) in use being held atground or at a fixed potential relative to ground, and the pulsed oralternating or direct potential applied across the neighbouring pairs ofthe electrodes(61) in the array being respectively above and below thatof the continuous electrode (14,15,16), whereby, in addition to surfacedischarge across the said surface between adjacent pairs of electrodes(61) in the array there is a volume plasma discharge between the saidcontinuous electrode (14,15,16) and the said array of electrodes (61).10. A reactor as claimed in claim 9, further characterised in that astacked arrangement of components provides a plurality of gas flow paths(32 a, 32 b, 32 c; 33 a, 33 b, 33 c) in parallel, each component in thestacked arrangement comprising a gas flow path defined between a saidcontinuous electrode (14,15,16), a said surface and a said array ofdiscrete electrodes(61) positioned close to the said surface, the gasflow path extending over and through the full extent of the surfaceacted upon by the said array of discrete electrodes (61).
 11. A reactoras claimed in any of claims 7 to 10, wherein concentration of highelectric field in regions close to the said surface and adjacent to theboundary gas flow region enhances trapping of particulate material orelectrically charged species by electrostatic forces at the said surfaceand enhances energy transfer into gas reactions in this region.
 12. Areactor for the plasma assisted treatment of a gaseous medium comprisinga pair of electrodes and a gas flow path between the electrodes,dielectric material in the gas flow path between the electrodes in theform of one or more barrier layers and/or a packing of discretecomponents such as beads, spheres or the like, characterised in that thesurface or surfaces of at least a part of the dielectric materialexposed to gas in the gas flow path is provided with discontinuous,isolated, thin metal film deposits.
 13. A reactor as claimed in claim12, further characterised in that the thin film metal deposits are inthe form of dots or spots on the surface on which they are deposited.14. The use of a reactor as claimed in any of the preceding claims forreforming hydrocarbon-based liquid or gaseous fuel.